1. Technical Field of the Invention
The present invention relates in general to the mobile telecommunications field and, in particular, to a method and apparatus for a packet switched system to emulate a locating algorithm of a circuit switched system in a cellular communications system, or vice versa.
2. Description of Related Art
Generally, the use of packet switching in mobile communications systems will provide operators with a versatile platform for a large variety of data applications. In fact, communications system developers anticipate that packet switched communications will form a significant part of the future's mobile telephony traffic. Consequently, it is important to ensure that future packet switched systems will be capable of operating effectively in a wide range of communications environments. It is also important to ensure that future packet switched systems will be developed with a high degree of network design flexibility.
In all existing circuit switched mobile systems, control over cell reselection in the idle mode is exercised by a relatively simple software algorithm executed in a processor located in the mobile station. However, in most of these systems, control over cell reselection in the active mode (locating) is exercised by a more advanced, highly flexible software algorithm executed by a processor located in the fixed network. In contrast, for all packet switched mobile systems, control over cell reselection in the idle, standby and active modes is exercised by a relatively simple software algorithm in the mobile station.
Cell planning for circuit switched mobile systems has to be conducted with respect to the active mode cell selection algorithm (locating), since it is the task of the locating algorithm with its controlling parameters to realize the intentions of the cell plan. In other words, the active mode traffic creates the radio network environment in terms of geographical signal strength and interference distribution.
On the other hand, cell planning for a packet switched mobile system has to be conducted with respect to either the idle mode or standby mode cell reselection algorithm. Since packet traffic is very "bursty" (i.e., the majority of packet switched data transfers occur during relatively short periods of time), there is generally too little time available (translating to excessive signalling) to make it worthwhile to change cells by the locating process, when changing from the idle mode to the active mode. Therefore, the active transmissions occur in the cell that has been selected by the idle/standby cell reselection algorithm.
These two different methods used for performing radio network control can lead to different results for various cell selection evaluations made. These different results, in turn, give rise to different radio network environment characteristics for the two types of systems, such as, for example, different handover borders, or different geographical distributions for average signal interference.
If a packet switched system and a circuit switched system share a frequency band in the same geographical area, each system's mobile station population (or type of traffic) can contribute to increased radio signal interference with the other system's mobile station population (or type of traffic). This interference originates in those areas where the handover borders differ. Consequently, a circuit switched mobile system operating with a "tight" frequency plan may not be able to accommodate packet data channels on the same carrier frequencies unless the "tight" frequency plan is relaxed.
In such a multi-service environment, the packet switched system is typically an add-on to an existing circuit switched system having a relatively large subscriber base. Consequently, the amount of packet switched traffic being carried is relatively small compared to the circuit switched traffic. Therefore, it follows that a higher percentage of packet switched traffic than circuit switched traffic will be susceptible to signal interference. This interference occurs primarily because the cell borders for the packet switched traffic and the cell borders for the circuit switched traffic differ from each other due to the differences in the cell selection algorithms for the two types of traffic.
In existing cellular communications systems, advanced locating control is exercised either by a network entity (or entities), or simple cell reselection is used by the mobile stations. For example, in the Nordic Mobile Telephone (NMT) System, Total Access Communications System (TACS), Advanced Mobile Phone System (AMPS), Digital Advanced Mobile Phone System (D-AMPS), Global System for Mobile Communications (GSM), Personal Digital Cellular (PDC) System, and IS-95 Code Division Multiple Access (CDMA) System, one or more network entities exercise advanced locating control in the active mode, while the mobile stations perform simple cell reselection in the idle mode. In the Digital European Cordless Telephone (DECT) and IS-661 circuit switched systems, the mobile stations perform simple cell reselection in both the active and idle modes. Furthermore, in the Cellular Digital Packet Data (CDPD) and Mobitex packet switched systems, the mobile stations perform simple cell reselection in the active, standby and idle modes. In other words, as demonstrated by the differences described above, there is no existing cellular communications system that utilizes a cell reselection algorithm that emulates any other cell reselection or locating algorithm.